The present invention relates to an ATM data controller to be provided between an ATM (Asynchronous Transfer Mode) interface and a data processing system to be connected to the ATM interface, for controlling data exchange between the ATM interface and the data processing system, and particularly to the ATM data controller having a function for compressing and decompressing data to be exchanged between them.
In a Japanese patent application entitled "A Relay/Exchange System of Compressed Vocal Data in an ATM Network", laid open as a Provisional Publication No. 055753/'97, there is disclosed a prior art for reducing quality degradation and relaying delay of vocal data in a relay/exchange system connected to an ATM network wherein the vocal data are exchanged being compressed into low bit-rate digital data.
FIG. 8 is a block diagram illustrating the relay/exchange system of the prior art.
Referring to FIG. 8, a voice terminal 30, such as a telephone, is connected to an ATM network by way of a digital exchange 20, and a cell assembling/disassembling unit 10 is provided between the digital exchange 20 and the ATM network, which takes charge of compression/decompression of the vocal data together with assemble/disassemble of ATM cells. A cell assembling/disassembling unit having the same configuration with the cell assembling/disassembling unit 10 is provided for each pair of trunk lines (not depicted in the drawings) of the digital exchange 20.
The cell assembling/disassembling unit 10 comprises a CDV (Cell-Delay-Variation) absorbing buffer 111 for buffering ATM cells received from the ATM network, a CDV absorption controller 112, a cell disassembling section 113, a decompression section 114 and a first selector 115. The cell disassembling section 113 reproduces compressed digital vocal data by disassembling ATM cells read out from the CDV absorbing buffer 111. The decompression section 114 decompresses the compressed digital vocal data into digital vocal data. The first selector selects data to be transferred to the digital exchange 20 from either of the digital vocal data outputted from the decompression section 114 or the ATM cells directly read out from the CDV absorbing buffer 111.
The cell assembling/disassembling unit 10 also comprises a compression section 121, a cell assembling section 122, a second selector 123, and a relay/exchange detection section 131. The compression section 121 compresses digital vocal data supplied from the digital exchange 20 into compressed digital vocal data of a low bit rate, which is assembled into ATM cells by the cell assembling section 122. The second selector 123 selects data to be transmitted to the ATM network from either of ATM cells assembled by the cell assembling section 122 or the data directly supplied from the digital exchange 20.
The relay/exchange detection section 131 takes charge of discriminating whether the data supplied from the digital exchange 20 is the data originated from a voice terminal 30 accommodated in the digital exchange, or the data relayed by another digital exchange by way of the ATM network, and controls the first and the second selector 115 and 123 according to the discrimination result.
When the data supplied from the digital exchange 20 is found to be the data relayed from another digital exchange through the ATM network, the relay/exchange detection section 131 controls the first selector 115 to select ATM cells directly read out from the CDV absorbing buffer 111 and the second selector 123 to select the data supplied directly from the digital exchange 20, because the ATM cells received from the ATM network are also to be relayed to the other digital exchange through the ATM network.
Thus, unnecessary decompression and compression of the vocal data to be relayed by the digital exchange 20 from the ATM network to the ATM network are avoided, and the quality degradation and the relaying delay of the vocal data are prevented, in the relay/exchange system according to the prior art.
However, there are problems in the prior art of FIG. 8, when it is applied as an ATM data controller for controlling data exchange between an ATM interface and a data processing system to be connected to the ATM interface.
A first problem is that the transmission efficiency of the ATM network cannot be made high because the cell assembling/disassembling unit 10 of FIG. 8 has no CDV-absorbing buffer for buffering ATM cells to be transmitted to the ATM network. When the data supplied from the digital exchange 20 is the digital vocal data originated from the voice terminal 30, it is assembled into ATM cells after compressed by the compressing section 121 according to the recommendation G. 728 of the ITU-T standard, for example. Therefore, a time interval for the data compression is needed after a cell is transmitted until a next cell is prepared, which makes low the transmission efficiency of the ATM network.
A second problem is that operational load of a CPU of the ATM data controller becomes high, because such data compression/decompression as above described is usually implemented by a software program executed by the CPU, when the ATM data controller performs the data compression/decompression between the data processing system and the ATM interface. Therefore, the operational load of the CPU taken by the data compression/decompression may obstruct other processing to be performed by the CPU.
A third problem is that transmission rate of data other than the vocal data is limited within the cell-transfer-rate of the ATM network, because the cell assembling/disassembling unit 10 of FIG. 8 has 110 means for compressing or decompressing data other than the digital vocal data.